Enthalpy and Volume of Ideal Solutions

We have stated above that the enthalpy and volume change involved in forming an ideal solution from its pure components is zero (equations (10.6) through (10.8)). This makes good intuitive sense and could be given as a definition or criterion of ideal solutions. Both conditions follow from equation (10.1), which is all that is needed to define ideal solutions. 

Considering the volume first, differentiate (10.1) with respect to pressure  

This shows that the partial molar volume Vi of a component of an ideal solution is equal to the partial molar volume of that component in its pure state, V°, which of course is also equal to the molar volume in the pure state, V°. It follows that there is no volume change when the pure components dissolve in one another to form the solution, i.e. 

It also follows that the volume of an ideal solution is the same as that of a mechanical mixture of the components (a mixture in which no dissolution takes place), and because the volume of such a mixture is 

The lack of volume change on ideal dissolution will be referred to in Chapter 11 ( 11.9.4) as Amagat s Rule. 

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